In automatic switched optical communications networks, comprising a plurality of network nodes with interconnecting links between respective pairs of nodes, it is necessary for network domains within the network to exchange topology information; in automatic switched transport networks (ASTN) this is carried out using a routing protocol such as optical shortest path first (OSPF) or intermediate system to intermediate system (ISIS). The topology information is required by path computation elements within the network domains for computing paths through the network, including alternative paths to restore failed circuits in case a fault is detected in the network and the fault affects the route of one or more signaled paths. During a path computation of a new route through a network, with certain Traffic Engineering characteristics and technology related limitations, the reachability of resources is taken into account together with the characteristics of each interconnecting link reported by the routing protocol.
The time required to carry out a path computation is a function of the number of nodes and interconnecting links in the network. The paths may be determined using a modified Dijkstra algorithm, using the Landau notation, which has a running time equal to O(L2), where L is the number of interconnecting links. This means that in big networks the running time of the algorithm can become long, causing higher traffic loss intervals and resulting in the non-achievement of Service Level Agreements stipulated between a network operator and its customers.
In addition, ASTNs owned and operated by different operators are often required to be interconnected, and each operator needs to disclose network topology information to the other operators in order to enable the other operators to route traffic through that network. It is often commercially desirable not to have to disclose network topology information to other network operators in order not to reveal business critical information, particularly regarding the physical structure of the network.